1. Field of the Invention
The present invention relates to the field of thermal machines. It relates to an inner casing of a rotating thermal machine.
2. Brief Description of the Related Art
Inner casings of a steam turbine are usually composed of two half shells, specifically an upper and a lower casing half which are screwed to one another in the horizontal parting plane by means of flanges which are arranged on the shells (see for example document U.S. Pat. No. 6,273,675 B1).
It is known that various problems may occur owing to the high temperatures occurring at the casing and the flanges which are screwed to one another: the casing is mechanically very rigid in the region of the flanges while it is comparatively weak at the poles which do not have flanges. As a result, it is possible for oval deformations (in cross section) to occur, i.e., radial deformations which result in grinding of the tips of the blades which rotate in the interior against the casing walls or a large amount of play between the blade tip and the casing wall, which adversely affects the efficiency.
In the past, proposals have already been made to reduce the radial deformations by means of a specific configuration of the casing while retaining the flanges (U.S. Pat. No. 6,336,789 B1). These solutions require additional space and additional material but basically do not change the asymmetry caused by the flanges.